Impulse excited magnetic deflection system



April 24, 1956 A. w. FRIEND 2,743,392

IMPLUSE EXCITED MAGNETIC DEFLECTION SYSTEM Filed Nov. 25, 1953 INVEN'I'OR.

United States Patent- IMPULSE EXCITED IVIAGNETIC DEFLECTION SYSTEM Albert W. Friend, Bala-Cynwyd, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application November 25, 1953, Serial No. 394,259 11 Claims. (Cl. 31527) This invention relates to wave generating systems, and more particularly to systems for generating sawtooth waveform currents.

There are many fields in which it is desirable to generate electric currents or electromagnetic waves which linearly increase to a maximum point and then decrease to a minimum point. Such waves are known as sawtooth waves. By way of example, among the many useful fields of application of such waves, are varieties of precision timing circuits, switching circuits, and as a time base or sweep for a cathode ray tube, as for example, in television applications. While this invention is readily adaptable to all such fields, for purposes of simplicity the description will be limited to the use of such a gen erator for providing a sawtooth current wave as required in a magnetically deflected cathode ray tube operated in a television system. In such systems generally, the current is supplied to a coil which is employed to create a magnetic field across the neck of the tube that varies linearly with time. The magnetic field, in turn, produces a proportional deflection of the electron beam.

Early forms of sawtooth wave generators had a relatively low efficiency, because a damping action was desirable .to give the waveform linearity. This necessitated an energy discharge path and the energy so dissipated in the damping circuit became lost. Attempts to increase the efliciency successfully utilized gaseous conduction tubes, such as the so-called thyratron, rather-than high vacuum tribes; however, this step alone did not give the desired efficiency. A further improvementwas made in several systems in which the energy normally dissipated in the damping circuit was capacitively stored and employedto effect a boost in the B supply voltage. Such systems greatly' -i'rnproved the operatingefliciency of deflection circuits as well as desirably increasing the effective ,B supply voltage. The circuits used to effect the so-called B boost action, however, required expensive circuit components, which ordinarily included a deflection coupling transformer. These components not only add to the cost of the deflection system but also-incur certain power losses in their operation.

The B boost action effected a higher available deflecting voltage; and inaddition a further increase of voltage was gained by utilizing a series resonant circuit between the power supply and the wave generator. This voltage increase was brought about by the fact that, in a resonant circuit having series inductance and capacitance, when it isperiodically charged from a D. C. source, the peak chargingsurge voltage across the capacitance is approximately equal to almost twice the applied voltage.

The present invention, in its more general form, contemplate's the use of a resonant circuit to generate a voltage'of: higher magnitude than the voltage of supply. This higher voltage, which is built up across the capacitance of the resonant circuit, is then supplied to an inductance wherein it generates a linear current wave of sawtoothform by reason of a discrete timing operation wherein energy in the inductance is provided with a dis- 2,743,392 Patented Apr. 24, 1956 charge path through a damping circuit. In addition, the timing operation and circuitry make it possible to utilize the damping circuit as a means of transfer for energy back to the resonant circuit, thereby effecting a B boost action and a saving of energy.

It is, therefore, an object of the present invention to provide an improved high efficiency, low cost linear sawtooth wave current generator.

Another object of this invention is to provide an improved television magnetic deflection generator.

A further object of this invention is to provide a saw tooth wave current generator of the B-boost type in which a transformer is not required.

Other and incidental objects of the invention will be apparent to those skilled in the art from reading the following specification and on inspection of the accompanying drawings, in which:

Figure 1 shows a circuit diagram of a preferred embodiment of this invention; and,

Figure 2 shows curves representative of wave forms of voltages and currents generated within the circuit of Fig. 1.

Referring to Fig. 1, there is shown a pair of power input terminals, 10 and 12, adapted to be connected to a suitable source of power, not shown. Serially connected between the power terminals, 10 and 12 respectively, in the following order, are: an inductor 14, a gaseous diode 18, and a storage capacitor 16. The gaseous diode 18 has a plate 28, which is connected to the inductor 14, and a cathode 30, which is connected to the storage. capacitor 16 so as to pass current from the inductor 14 to the storage capacitor 16. Shunting the gaseous diode 18 is a series circuit comprising a voltage booster capacitor 24 and a deflection yoke coil 20. The

gaseous diode 18 has its plate connected to the voltage booster capacitor 24 and its cathode connected to the deflection yoke coil 20. A thyratron 22, having a plate 32, a cathode 34, and a control grid 36, has its plate 32 connected to a point between the voltage booster capacitor 24 and the deflection yoke coil 20, and has its cathode connected to input terminal 12. The control grid 36 is connected to a synchronizing or timing pulse input terminal 38.

An understanding of the operation of the circuit of Fig. 1 may be furthered by considering the circuit with reference to the curves of Fig. 2, during a cycle of operation. The curves shown in Fig. 2 are approximate representations of voltage and current magnitudes plotted with time as the abscissa. Curve A represents the varying voltage across storage capacitor 16, and curve B the varying current through inductor 20. Time T represents the time of the initial application of a voltage at the power-input terminals 10 and 12. In Fig. 1, at the time T, there is no closed current path, however, shortly thereafter a small voltage builds up across the gaseous diode 18 causing it to break down, thereby closing a circuit from the input terminal 10 through the inductor 14, the gaseous diode 18, the storage capacitor 16, and back to the input terminal 12. The current path during this time is effectively a series circuit of inductance and capacitance, and the values of the storage capacitor 16 and the inductor 14 are so chosen as to make this series circuit resonant at a frequency approximately one-half the desired repetition rate of electron beam scanning. Current through the inductor 14 now commences to charge capacitor 16, substantially sinusoidally and, due to the fact that energy stored in the inductor 14 adds to energy from the applied attains will, of course, depend on the characteristics of the series circuit, particularly the circuit Q.

In Fig. 2 the above-described charging period is represented graphically by the curve between T and To. During this period the voltage booster capacitor 24 and the deflection yoke coil 20 play no part in the circuit operation, due to the fact that the only voltage applied to them is the ionization potential of the gaseous diode 18, which, for practical purposes, may be ignored due to its small magnitude. At the time when the voltage across the capacitor 16 reaches a maximum, the current through the gaseous diode 18 approaches zero and, due to the small voltage now existing across the diode it is extinguished. As the voltage builds up in a reverse direction across the diode it will, of course, pass no current back to the inductor 14. It is to be noted, therefore, that the function of the diode could be performed by any unilateral conduction device, or a suitable timed switch. Nearly simultaneously with the extinguishing of the diode a pulse is applied to the pulse input terminal 38, and thence to the control grid 36 of the thyratron 22. The application of this pulse fires the thyratron 22, and the active portion of the circuit may now be considered as including only the storage capacitor 16which is charged-the deflection yoke coil 20, and the thyratron 22. The inductor 14 and the voltage booster capacitor 24 may carry current during this period, however, due to the states of the gaseous diode 18 and the thyratron 22 acting as switches, the only efi'ect will be to tend to place a charge on discharge capacitor 24. The charge on storage capacitor 16 is provided with a discharge path through deflection yoke coil 20 and thyratron 22. The instant this path is provided, the voltage across capacitor 16 will be equal to the voltage across the deflection yoke coil 20, plus the voltage across the thyratron 22 by reasons of the fact that the closing of thyratron 22 forms a closed loop or current path, including capacitor 16, deflection yoke coil 20, and thyratron 22. The fact that the sum of the algebraic voltage drops around the closed loop equal zero coupled with the fact that the thyratron 22 offers small resistance to current and therefore has a small voltage drop, indicates that the voltages across storage capacitor 16 and deflection yoke coil 20 are nearly equal, but

opposite.

In Fig. 2, To indicates the time at which the diode 18 stops conducting, and the thyratron 22 begins to conduct. 7 The voltage across the storage capacitor 16 will decrease exponentially as the charge leaks off through deflection yoke coil 29 and thyratron 22 as'shown in curve A betwen To and T1. The leakage of charge during this time creates a current through the coil which increases in magnitude to a maximum, as shown in curve B of Fig. 2 during the same time interval. At the time T1 when the maximum current point is reached, the voltage across the storage capacitor 16 is beginning to swing negative, due

to the collapse of magnetic lines of force about the deflection yoke coil 20. This negative voltage, coupled with the applied voltage, fires the diode 18, thereby lowering the anode to cathode voltage of the thyratron 22 and rendering that tube in effect non-conducting. The active circuit now becomes similar to what it was when the gaseous diode 18 began conducting, shortly after the initial application of a voltage to the power terminals and 12, with the exception that the deflection yoke coil is now carrying a current. The deflection yoke coil 20. by reason of its inductive nature, will attempt to sustain any current passing through it by releasing energy stored in its magnetic field. Therefore, .a gradually decreasing current passes through a damping circuit containing voltage booster capacitor 24. Repeated cycles charge voltage booster capacitor 24 to a limiting value, with a polarity of boost voltage (EB) which adds to the supply voltage (Es) toproduce a total equivalent supply veltage'E=Es+EB, thus aiding the flow of current created by the voltage applied at input terminals 10 and 12. I

The capacitor 16 is therefore, charged to an increased potential slightly less than EI+EB- The charge, so placed on capacitor 16, by the higher voltage, is conserved and augments the discharge current through the deflection yoke coil 20. This is an advantageous result of the so callcd B-boost effect caused by the voltage across voltage booster capacitor 24, acting to increase the charge of retrace capacitor 16. During the next period, when the thyratron 22 conducts, the charge placed on capacitor 24 will be discharged sufficiently to pass the current from the deflection-yoke coil 20 during the following cycle.

, The interval T1 to To indicates the time period of the decreasing current in the'deflection yoke coil 20 as shown in curve.B during which the gaseous diode 18 conducts and the thyratron 22 is cut off. The gradually decreasing current is made essentially linear by the action of the discharge circuit in providing a nearly constant potential drop across the gaseous discharge of diode 18 and the large reservoir of electrostatic charge of capacitor 24. As the cycle is repeated with furtherapplication of synchronizing pulses the values of voltage and current attained in the operation will become stabilized as a limiting condition is reached. Curve B of Figure 2 will, therefore, approach a uniform sawtooth waveform.

From the foregoing, it can be seen that the applicant has provided an improved form of a sawtooth wave generator.

What is claimed is:

l. A sawtooth wave generator comprising a series resonant circuit having inductance and capacitance, adapted to receive a potential; a unilateral conduction device connected betwen said inductance and said capacitance, said device being so connected as to pass current from said inductance to said capacitance, the conductive polarity of said device being such as to concur with the voltage potential applied to said circuit, a circuit in shunt with said capacitance and said unilateral conducting device, including a condenser having one terminal connected to the junction point of said inductance and said unilateral conducting device, and a switching device connected to the other terminal of said condenser, an inductive circuit con-. nected between the junction of said unilateral conducting device and said capacitance, and the other terminal of said condenser.

2. Apparatus according to claim 1, wherein said unilateral conductive device comprises a gas diode.

3. Apparatus according to claim 1, wherein said switch comprises a thyratron having an anode, a cathode, and a. control grid, said control grid being adapted to receive synchronizing pulses to effect a switching operation.

4. Apparatus according to claim 1, wherein said inductive circuit comprises a deflection yoke coil.

5. A sawtooth wave generator, comprislng'a resonant" circuit, including capacitance and inductance, adapted to be energized, means to cyclically prevent the flow of current from said capacitance to said inductance, an inductive circuit, connecting means between said resonant circuit and said inductive circuit, said connecting means having switching means to periodically allow the transfer of a part of the energy stored in said resonant circuit to said inductive circuit, at a time when there is no flow of current between said capacitance and said inductance, a condenser connected between said inductive circuit and said resonant circuit, said switching means operating to periodically allow the transfer of a part of the energy 1 stored insaid inductive circuit back to said resonantcircuit through said condenser at a tirnewhen there is current flow between said capacitanceand said inductance.

6. Apparatus according to claim 5 wherein said means to interrupt the flow of current comprises a unilateral conducting device.

7. Apparatus according to claim 5 wherein switching means comprises a thyratron circuit adapted to perform the switching operation upon the application of a synchronizing pulse.

8. Apparatus according to claim 5 wherein said inductive circuit comprises a deflection yoke coil.

9. An electromagnetic deflection generator for a cathode ray system comprising a deflection yoke coil, a gaseous diode, having a plate and a cathode, a first condenser, said cathode of said gaseous diode being connected to one terminal of said deflection yoke coil, said plate of said gaseous diode being connected to the other terminal of said deflection yoke coil through said first condenser, a second condenser connected to said cathode of said gaseous diode, an inductance connected to the plate of said gaseous diode, a gaseous conduction device having a plate, a cathode, and a control grid, said plate of said gaseous conduction device being connected to said other terminal of said deflection yoke coil, the free terminal of said second condenser being connected to the cathode of said gaseous conductive device, means for applying a source of power between the free terminal of said inductance and said cathode of said gaseous conduction device, means for applying periodically recurrent timing pulses to said control grid in such polarity to fire said second gaseous discharge device.

10. In a device of the character described having a series resonant circuit comprising capacitance and inductance, adapted to be energized; an inductive circuit; and switching means; wherein said switching means periodically acts to transfer energy from said resonant circuit to said inductive circuit for the purpose of generating a sawtooth current wave; the improvement which consists of a capacitive circuit, connected between said inductive circuit and said resonant circuit, a unilateral conduction device connected between said capacitance and said inductance of said series resonant circuit, said capacitive circuit functioning to periodically store energy and transfer it back to said resonant circuit during the time interval when said unilateral conduction device is passing a current.

11. A sawtooth wave generator comprising a resonant circuit including capacitance and inductance adapted to be energized, an inductive circuit; switching and connecting means to periodically transfer energy directly from said resonant circuit to said inductive circuit; a capacitive circuit connected between said resonant circuit and said inductive circuit; said switching means periodically acting to transfer a part of the energy stored in said inductive circuit to said capacitive circuit and thence back to said resonant circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,445,017 Boadle et a1 July 13, 1948 2,545,346 Edelsohn Mar. 13, 1951 2,552,884 Cannon May 15, 1951 2,565,392 Neuwirth Aug. 12, 1951 2,571,824 Boyd et a1. Oct. 16, 1951 2,596,590 Overton May 13, 1952 2,606,305 Court Aug. 5, 1952 

